1. Field of the Invention
The present invention relates generally to semiconductor wafer preparation and more specifically to in situ metrology for process parameter control during wafer processing.
2. Description of the Related Art
As is well known, semiconductor fabrication includes several stages during which an underlying substrate is subjected to the formation and removal of various layers. The continuous demand for smaller feature sizes and tighter surface planarity in conjunction with the constant quest to increase wafer throughput requires implementing a process state monitoring and endpoint detection method capable of discontinuing the processing of a target layer once a target thickness has been achieved.
Recently, eddy current sensors (ECS) are starting to be used to measure changes in film properties. For more information on these new methodologies for using eddy current sensors, reference may be made to “INTEGRATION OF EDDY CURRENT SENSOR BASED METROLOGY WITH SEMICONDUCTOR FABRICATION TOOLS” having the U.S. patent application Ser. No. 10/186,472, filed on Jun. 28, 2002. The disclosure of this patent application is incorporated herein by reference. The ECS sensors rely on the induction of a circular current in a sample by the fluctuating electromagnetic field of a test coil proximate to the object being probed. Fluctuating electromagnetic fields are created as a result of passing an alternating current through the coil. The fluctuating electromagnetic fields induce eddy currents, which perturb the applied field and change the inductance of the coil.
FIG. 1 is a simplified schematic diagram of the principle upon which an eddy current sensor operates. An alternating current flows through coil 108 defined in close proximity to the conducting object 102. The electromagnetic field of the coil 108 induces eddy currents 104 in conducting object 102. The magnitude and the phase of the eddy currents 104 in turn affect the loading on the coil 108, causing the impedance of the coil 108 to be impacted by the eddy currents 104. This impact is measured and calibrated in terms of proximity of conducting object 102 and/or thickness of the object 102 if the thickness of the object 102 is significantly less than the field penetration depth. As can be seen, distance 106 impacts the effect of eddy currents 104 on coil 108. As such, if object 102 moves, the signal from the sensor monitoring the impact of eddy currents 104 on coil 108 will also change.
In a chemical mechanical planarization (CMP) operation, a wafer carrier includes an isolated built-in eddy current sensor for measuring the thickness of the thin film layer being processed during the CMP operation. The wafer carrier includes a carrier film designed to support the wafer. During the planarization operation, the rotating carrier, the built in eddy current sensor, and wafer are pressed against the polishing pad, planarizing the surface of the wafer.
Unfortunately, using eddy current sensors for detecting an endpoint of the target layer or measuring the thickness of the target layer has certain negative aspects. For instance, the plot 200 shown in FIG. 2 depicts the eddy current sensor signals generated in a center and edge of a wafer. A graph 114 shows the changes in eddy current voltage versus time during the planarization operation. In graph 114, changes in eddy current voltage is sensed by an isolated eddy current sensor defined in the center of the wafer while a graph 116 shows the changes in eddy current voltage during the planarization operation sensed by another isolated eddy current sensor defined in the edge of the wafer. Normally, the eddy current sensor signals undulate sinusoidally, with each signal undulation following a frequency of the carrier rotation. As shown in FIG. 2, however, despite both signals undulating sinusoidally, the signal amplitude in the edge graph 116 is shown to be considerably higher than the amplitude in the center graph 114.
Furthermore, the probed thin film layer allows the electromagnetic field to penetrate the thin film layer so as to reach conductive objects located in the sensing vicinity. Generally, the configuration of the external objects is asymmetric with regard to the trajectory of the rotating sensor. However, rotational proximity variation results in sinusoidal variation in the signal amplitude attributed to rotation of the wafer carrier and thus the eddy current sensors in a non-uniform external media.
The variation in the sinusoidal signal amplitude is caused by the sensitivity of the eddy current sensors to a wide spectrum of parameters. For instance, among many other parameters, it has been established that eddy current sensors are sensitive to variation in carrier film thickness, standoff, temperature, and pressure. Additionally, the magnitude and phase of the eddy current generated in the probed thin film layer is sensitive to the properties of the thin film layer (e.g., thickness, resistivity, topography, etc.) as well as thin film layer/sensor proximity.
By way of example, the “standoff” parameter, i.e., the distance between the layer to be polished and the eddy current sensor surfaces, may differ for a number of reasons. A substantial variation in the standoff is created when the carrier film thickness varies (e.g., between +/− a few mils). The standoff further varies as a result of changes in the thickness of the carrier film due to compression of the carrier film being applied to the polishing pad with different degrees of pressure. The thickness of the carrier film and thus the standoff furthermore changes once the leading edge of the rotating wafer digs into the moving polishing pad at the point of contact. At this point, the pressure applied at the point of contact causes the carrier film to be compressed, varying the standoff, and thus the amplitude of the eddy current signal. As can be appreciated, it is extremely difficult to calibrate for all the parameters affecting the standoff, which ultimately negatively impacts the thickness measurement by the sensor.
Another variable parameter affecting the eddy current signal amplitude is having non-uniform temperature gradience across the wafer surface. For instance, the temperature of the wafer leading edge increases as the wafer leading edge comes into contact with the moving polishing pad. Then, the temperature of the wafer trailing edge increases as the wafer trailing edge comes into contact with the polishing pad. The sensitivity of the eddy current sensor to variation in temperature, directly influences the eddy current sinusoidal signal. This again makes it extremely difficult to calibrate for temperature variances impacting the thickness measurement of the eddy current sensors.
Additionally, the sinusoidal signal amplitude differs depending on the sensor being defined within the wafer carrier close to the wafer center or the edge of the wafer. The signal amplitude increases as the sensors are defined further away from the wafer center.
As can be appreciated, the conjunctive effects of these parameters has introduced an unacceptably high amount of error and unpredictability into the thickness measurement or endpoint detection using the eddy current sensor signals, leading to under polishing or over polishing of the processed wafer layers, damaging the wafers and thus, reducing wafer throughput and yield.
In view of the foregoing, there is a need for a flexible methodology and system capable of determining a thickness of a target layer by controlling the process parameters.